1. Technical Field
This invention generally relates to a system for preparing dental prostheses. In particular, the invention relates a smart mill blank library and preparing dental prostheses for use as crowns, onlays, inlays, veneers, bridges, and other restorations from a mill blank selected from a mill blank library.
2. Related Art
The art of fabricating custom-fit prosthetics in the dental field is well-known. Prosthetics are replacements for tooth or bone structure. They include restorations, replacements, inlays, onlays, veneers, full and partial crowns, bridges, implants, posts, and the like. Typically, a dentist prepares a tooth for the restoration by removing existing anatomy, which is then lost. The resultant preparation may be digitized or a dental impression is taken, for the purpose of constructing a restoration. The restoration may be constructed through a variety of techniques including manually constructing the restoration, using automated techniques based on computer algorithms, or a combination of manual and automated techniques. In one known technique, the prosthetic is fabricated using a computer-assisted (CAD/CAM) system, such as a computer-aided milling machine. One such machine is the CEREC 3D system from Sirona Dental Systems. Computer-aided machines of this type work by shaping the prosthetic from mill blanks. A mill blank is a solid block of material from which the prosthetic is shaped by a shaping apparatus whose movements are controlled by the computer. Under computer control, the size, shape, and arrangement of the restoration may be subject to various physical parameters, including neighboring contacts, opposing contacts, emergence angle, and color and quality of the restoration to match the neighboring teeth.
A common restoration includes a porcelain-fused-to-metal (PFM) crown. The crown typically comprises a cap of porcelain material overlayed on a thin metal coping. The metal coping forms an interface between the preparation and the porcelain material. Common restorations typically include a coping formed from precious or semi-precious metals, including gold or a gold alloy. The material may be selected based on the color and various other properties to optimize a long-lasting natural looking restoration.
The copings or full metal crowns typically are formed from a lost wax casting process. The process may include placing several wax copings on a wax tree, which is connected to a wax base. The structure is placed in a cylinder with investing material, and the wax is melted out after the investing material has set. A molten metal, typically a gold alloy, is then poured into the remaining structure, and the entire cylinder is placed into a centrifuge to distribute the molten material to a uniform distribution. Preferably, the alloy base and the tree are recovered for use in a future casting process. The continued re-melting of the gold alloy along with other contaminants, however, introduces oxidation and other tarnishing agents into the gold alloy.
Other methods for forming the coping may be used, including milling or machining with some kind of block or blank, but these techniques may waste much of the metal material. The ratio of the volume of the final metal coping to the volume of a typical enclosing mill blank (a symmetric block or cylinder) is often very small such that much of the material may be wasted. As noted above, a common milling process includes forming the coping from a mill blank using a computer-assisted milling machine. The blank includes a sufficiently large rigid attachment so that it may be held solidly while the machining process is underway. A rectangular or cylindrical blank is commonly used, and the vast majority of material is removed via the machining process. U.S. Pat. No. 4,615,678 to Moermann et al. discloses a conventional mill blank of this type made of ceramic silica material. There are, of course, numerous other types of mill blanks available commercially.
The cost of recovering the wasted material often exceeds the cost of the material sought to be recovered. The object may be milled using a wet milling process, which typically results in the discarded material (including fine particles) being mixed with water or other cutting fluids. This is not a significant concern when the restoration is being formed using inexpensive materials; however, when utilizing expensive materials, such as gold, the issue of dealing with the recovery of the machined material may make the process prohibitively expensive. Indeed, the cost of the discarded materials in the case of precious or semi-precious materials is the single most important reason that prior art techniques have proven to be undesirable or cost prohibitive. Additional concerns are the time required to cut through the discarded material, as well as the additional wear and tear on the tools.
There have been a few incidental suggestions in the art to address this problem. Thus, for example, U.S. Pat. No. 4,615,678 teaches that the body portion of a mill blank can be formed in a way to minimize wear on and run time of the milling machine by being shaped initially to more closely resemble the final implant. An illustrative example is a blank for use in forming a two lobed inlay that includes a transverse groove in one side thereof. U.S. Published Patent Application 2003/0031984 to Rusin et al. illustrates a similar blank construction, and it further notes that blanks can come in a variety of shapes and sizes.
While these suggestions are useful, there remains a need in the art to provide improved mill blank configurations and assemblages that facilitate prosthetic milling operations in a manner to reduce material waste, reduce machining time, and to increase value.